The work will proceed in the following directions: 1. Preparation of 57Fe labeled aconitase from a microorganism grown on 57Fe for detailed Mossbauer studies of its Fe-S cluster in various states of the enzyme, EXAFS, CD and MCD studies of the enzyme. Attempts to spin label aconitase with a SL-SH blocking reagent and to produce a spin labeled substrate analog. 2. Further studies by EPR at different frequencies of an early EPR detectable reoxidation product of reduced cytochrome c oxidase. Comparison of various described early reaction products of this reaction by using a combination of rapid optical scanning and freeze-quenching EPR. Mass ratio studies of H2O formed in the reoxidation of cytochrome oxidase by 16O2 and 18O2. 3. Further studies on the kinetics of reduction of flavin and Fe-S cluster by substrate in the reaction of bacterial trimethylamine dehydrogenase and trimethylamine by optical and EPR spectroscopy. 4. Further development of rapid mixing and freezing techniques with a view to multiple mixing procedures, subzero liquid aqueous-organic mixtures and the application of rapid optical scanning prior to freezing.